Digital Subscriber Line (DSL) technology greatly increases the digital capacity of an ordinary telephone line, allowing much more information to be channeled into a home or office receiver as well as providing for “always-on” operation. The speed that a DSL modem can achieve is a function of the distance between the receiver and a central office. Symmetric DSL (SDSL) is typically used for short connections that require high data rates in both upstream and downstream directions. SDSL utilizes only one cable pair and may be used with adaptive rates from 144 Kbps to 1.5 Mbps. High Bit Rate DSL (HDSL) is a symmetric technology that uses two twisted pairs for transmission distances up to about 12,000 feet. Each twisted pair can be used to provide T1 transmission, but such lines are typically not shared with analog telephone devices. Alternatively, HDSL-2 technology requires only one cable pair and provides transmission distances of about 18,000 feet.
Asymmetric DSL (ADSL) technology uses frequencies that are higher than voice, shares telephone lines, and is suitable for accessing the Internet. For ADSL applications, a Plain Old Telephone System (POTS) splitter may be installed at the receiver end to separate the voice frequencies and the ADSL frequencies. The ‘G.lite’ version of ADSL, alternatively known as ‘ADSL lite,’ ‘Universal ADSL,’ or ‘splitterless ADSL,’ does not require a POTS splitter. Rather, telephone devices are connected to low-pass filters that remove the higher ADSL frequencies from the lower-frequency voice transmissions. In the current state of the art, ADSL is using Discrete Multi-tone (DMT) modulation and Carrierless Amplitude Phase (CAP) modulation.
Cables comprising twisted-pair transmission channels emanate from essentially all telephone company central offices. Such cables typically include binders containing groups of twenty-five or fifty tightly-packed twisted pairs. As can be appreciated by one skilled in the relevant art, this configuration results in crosstalk among twisted pairs in the same binder. There are nearly a billion such twisted pairs worldwide and the number is growing dramatically as nearly all new commercial and residential building developments continue to install twisted-pair lines for telephone service connection. This “installed copper base” of twisted pairs lines may indeed be the greatest infrastructure asset of the local telephone service provider.
In view of the fact that DSL technology can provide for many services over this installed copper base, the communication lines in a binder can be viewed as a service provider resource, much as the wireless spectrum is viewed as a resource. This resource needs fair management to provide the maximum competitive financial return and economic prosperity to all stakeholders in the broadband marketplace. Thus, in view of the valuable resource provided by the binder, unstructured use of the binder communication lines at the physical layer by uncoordinated competitive service providers becomes detrimental to the overall binder data carrying capacity.
In a typical binder containing twenty five or fifty twisted copper-pair lines, insulation between the lines provides only minimal shielding between copper loops in the high frequency band that is used by the various flavors of xDSL technologies. Accordingly, as the deployment of the various xDSL products has grown, the resulting interference into neighboring loops sharing the binder also continues to increase. Consequently, it has become clear that ever-increasing levels of interference in the binder present a limitation to xDSL product performance. The cross-talk caused by the signals transmitted on the loops, such as self-echo, near-end cross-talk (NEXT), and far-end cross-talk (FEXT), is dependent on the transmit signal power levels. In conventional xDSL applications, this cross-talk reduces the signal-to-noise ratio (SNR) since the cross talk acts as additional noise to the receiver.
In DMT-based ADSL modems, a selected bandwidth of 1.104 MHz is divided into 256 bins and the data bits are used for Quadrature Amplitude Modulation (QAM) in each bin. In DMT-based VDSL modems, a selected bandwidth of 17.664 MHz is divided into 4096 bins. Normally during the initialization period, each modem goes through a channel SNR estimation phase and data/power allocation phase, so as to maximize its own data rate. Because of the fact that each modem's transmission power leaks into other loops within the same binder, the leaked power adds to the noise detected by modems on the other loops. Moreover, if two modems happen to be sharing the same frequency band, then an increase in transmission power of one modem reduces the transmission data rate of the other shared-frequency-band modem. On the other hand, if the two modems were not sharing a common frequency band, then one modem's operation would have less of an adverse effect on the data rate of the other modem. As can be appreciated by one skilled in the art, if both modems were cooperating and jointly performed initial power allocation over available frequency bands, the resulting transmission data rate of the two modems would greater than the transmission data rates achieved if both modems were not cooperating.
Data rates can be increased utilizing a power control algorithm based on a single user as disclosed, for example, in a related technical article by Wei Yu, George Ginis, and John M. Cioffi, entitled “An adaptive multi-user power control algorithm,” T1E.4 Contribution 01-200R3, August 2001. The core of the disclosed adaptive multi-user power control algorithm is an iterative water-filling procedure that effectively allocates transmission power among modem users and frequency bands when the power budget of each modem user is known. In the iterative water-filling procedure, multiple users take turns to allocate power based on the well-known water-filling method for a single user, taking into account the background noise and the cross-talk noise from other users, until the power allocation profile converges to equilibrium.
Thus, there is a particular need for a method which takes into account cross-talk among users in maximizing the total data rate achievable within a multi-carrier binder.